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<div class="section" id="module-eventlet.event">
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<span id="event-cross-greenthread-primitive"></span><h1><code class="xref py py-mod docutils literal"><span class="pre">event</span></code> – Cross-greenthread primitive<a class="headerlink" href="#module-eventlet.event" title="Permalink to this headline">¶</a></h1>
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<dl class="class">
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<dt id="eventlet.event.Event">
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<em class="property">class </em><code class="descclassname">eventlet.event.</code><code class="descname">Event</code><a class="headerlink" href="#eventlet.event.Event" title="Permalink to this definition">¶</a></dt>
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<dd><p>An abstraction where an arbitrary number of coroutines
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can wait for one event from another.</p>
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<p>Events are similar to a Queue that can only hold one item, but differ
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in two important ways:</p>
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<ol class="arabic simple">
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<li>calling <a class="reference internal" href="#eventlet.event.Event.send" title="eventlet.event.Event.send"><code class="xref py py-meth docutils literal"><span class="pre">send()</span></code></a> never unschedules the current greenthread</li>
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<li><a class="reference internal" href="#eventlet.event.Event.send" title="eventlet.event.Event.send"><code class="xref py py-meth docutils literal"><span class="pre">send()</span></code></a> can only be called once; create a new event to send again.</li>
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</ol>
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<p>They are good for communicating results between coroutines, and
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are the basis for how
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<a class="reference internal" href="greenthread.html#eventlet.greenthread.GreenThread.wait" title="eventlet.greenthread.GreenThread.wait"><code class="xref py py-meth docutils literal"><span class="pre">GreenThread.wait()</span></code></a>
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is implemented.</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">eventlet</span> <span class="k">import</span> <span class="n">event</span>
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<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">eventlet</span>
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<span class="gp">>>> </span><span class="n">evt</span> <span class="o">=</span> <span class="n">event</span><span class="o">.</span><span class="n">Event</span><span class="p">()</span>
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<span class="gp">>>> </span><span class="k">def</span> <span class="nf">baz</span><span class="p">(</span><span class="n">b</span><span class="p">):</span>
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<span class="gp">... </span> <span class="n">evt</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">b</span> <span class="o">+</span> <span class="mi">1</span><span class="p">)</span>
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<span class="gp">...</span>
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<span class="gp">>>> </span><span class="n">_</span> <span class="o">=</span> <span class="n">eventlet</span><span class="o">.</span><span class="n">spawn_n</span><span class="p">(</span><span class="n">baz</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>
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<span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
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<span class="go">4</span>
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</pre></div>
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</div>
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<dl class="method">
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<dt id="eventlet.event.Event.ready">
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<code class="descname">ready</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#eventlet.event.Event.ready" title="Permalink to this definition">¶</a></dt>
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<dd><p>Return true if the <a class="reference internal" href="#eventlet.event.Event.wait" title="eventlet.event.Event.wait"><code class="xref py py-meth docutils literal"><span class="pre">wait()</span></code></a> call will return immediately.
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Used to avoid waiting for things that might take a while to time out.
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For example, you can put a bunch of events into a list, and then visit
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them all repeatedly, calling <a class="reference internal" href="#eventlet.event.Event.ready" title="eventlet.event.Event.ready"><code class="xref py py-meth docutils literal"><span class="pre">ready()</span></code></a> until one returns <code class="docutils literal"><span class="pre">True</span></code>,
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and then you can <a class="reference internal" href="#eventlet.event.Event.wait" title="eventlet.event.Event.wait"><code class="xref py py-meth docutils literal"><span class="pre">wait()</span></code></a> on that one.</p>
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</dd></dl>
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<dl class="method">
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<dt id="eventlet.event.Event.send">
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<code class="descname">send</code><span class="sig-paren">(</span><em>result=None</em>, <em>exc=None</em><span class="sig-paren">)</span><a class="headerlink" href="#eventlet.event.Event.send" title="Permalink to this definition">¶</a></dt>
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<dd><p>Makes arrangements for the waiters to be woken with the
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result and then returns immediately to the parent.</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">eventlet</span> <span class="k">import</span> <span class="n">event</span>
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<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">eventlet</span>
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<span class="gp">>>> </span><span class="n">evt</span> <span class="o">=</span> <span class="n">event</span><span class="o">.</span><span class="n">Event</span><span class="p">()</span>
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<span class="gp">>>> </span><span class="k">def</span> <span class="nf">waiter</span><span class="p">():</span>
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<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'about to wait'</span><span class="p">)</span>
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<span class="gp">... </span> <span class="n">result</span> <span class="o">=</span> <span class="n">evt</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
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<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'waited for </span><span class="si">{0}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">result</span><span class="p">))</span>
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<span class="gp">>>> </span><span class="n">_</span> <span class="o">=</span> <span class="n">eventlet</span><span class="o">.</span><span class="n">spawn</span><span class="p">(</span><span class="n">waiter</span><span class="p">)</span>
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<span class="gp">>>> </span><span class="n">eventlet</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
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<span class="go">about to wait</span>
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<span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="s1">'a'</span><span class="p">)</span>
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<span class="gp">>>> </span><span class="n">eventlet</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
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<span class="go">waited for a</span>
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</pre></div>
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</div>
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<p>It is an error to call <a class="reference internal" href="#eventlet.event.Event.send" title="eventlet.event.Event.send"><code class="xref py py-meth docutils literal"><span class="pre">send()</span></code></a> multiple times on the same event.</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="s1">'whoops'</span><span class="p">)</span>
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<span class="gt">Traceback (most recent call last):</span>
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<span class="c">...</span>
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<span class="gr">AssertionError</span>: <span class="n">Trying to re-send() an already-triggered event.</span>
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</pre></div>
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</div>
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<p>Use <code class="xref py py-meth docutils literal"><span class="pre">reset()</span></code> between <a class="reference internal" href="#eventlet.event.Event.send" title="eventlet.event.Event.send"><code class="xref py py-meth docutils literal"><span class="pre">send()</span></code></a> s to reuse an event object.</p>
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</dd></dl>
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<dl class="method">
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<dt id="eventlet.event.Event.send_exception">
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<code class="descname">send_exception</code><span class="sig-paren">(</span><em>*args</em><span class="sig-paren">)</span><a class="headerlink" href="#eventlet.event.Event.send_exception" title="Permalink to this definition">¶</a></dt>
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<dd><p>Same as <a class="reference internal" href="#eventlet.event.Event.send" title="eventlet.event.Event.send"><code class="xref py py-meth docutils literal"><span class="pre">send()</span></code></a>, but sends an exception to waiters.</p>
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<p>The arguments to send_exception are the same as the arguments
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to <code class="docutils literal"><span class="pre">raise</span></code>. If a single exception object is passed in, it
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will be re-raised when <a class="reference internal" href="#eventlet.event.Event.wait" title="eventlet.event.Event.wait"><code class="xref py py-meth docutils literal"><span class="pre">wait()</span></code></a> is called, generating a
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new stacktrace.</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">eventlet</span> <span class="k">import</span> <span class="n">event</span>
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<span class="gp">>>> </span><span class="n">evt</span> <span class="o">=</span> <span class="n">event</span><span class="o">.</span><span class="n">Event</span><span class="p">()</span>
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<span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">send_exception</span><span class="p">(</span><span class="ne">RuntimeError</span><span class="p">())</span>
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<span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
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<span class="gt">Traceback (most recent call last):</span>
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File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
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File <span class="nb">"eventlet/event.py"</span>, line <span class="m">120</span>, in <span class="n">wait</span>
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<span class="n">current</span><span class="o">.</span><span class="n">throw</span><span class="p">(</span><span class="o">*</span><span class="bp">self</span><span class="o">.</span><span class="n">_exc</span><span class="p">)</span>
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<span class="gr">RuntimeError</span>
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||
</pre></div>
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</div>
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<p>If it’s important to preserve the entire original stack trace,
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you must pass in the entire <a class="reference external" href="https://docs.python.org/2/library/sys.html#sys.exc_info" title="(in Python v2.7)"><code class="xref py py-func docutils literal"><span class="pre">sys.exc_info()</span></code></a> tuple.</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">sys</span>
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<span class="gp">>>> </span><span class="n">evt</span> <span class="o">=</span> <span class="n">event</span><span class="o">.</span><span class="n">Event</span><span class="p">()</span>
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<span class="gp">>>> </span><span class="k">try</span><span class="p">:</span>
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<span class="gp">... </span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">()</span>
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<span class="gp">... </span><span class="k">except</span> <span class="ne">RuntimeError</span><span class="p">:</span>
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<span class="gp">... </span> <span class="n">evt</span><span class="o">.</span><span class="n">send_exception</span><span class="p">(</span><span class="o">*</span><span class="n">sys</span><span class="o">.</span><span class="n">exc_info</span><span class="p">())</span>
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<span class="gp">...</span>
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<span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
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<span class="gt">Traceback (most recent call last):</span>
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File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
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File <span class="nb">"eventlet/event.py"</span>, line <span class="m">120</span>, in <span class="n">wait</span>
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<span class="n">current</span><span class="o">.</span><span class="n">throw</span><span class="p">(</span><span class="o">*</span><span class="bp">self</span><span class="o">.</span><span class="n">_exc</span><span class="p">)</span>
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File <span class="nb">"<stdin>"</span>, line <span class="m">2</span>, in <span class="n"><module></span>
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<span class="gr">RuntimeError</span>
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</pre></div>
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</div>
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<p>Note that doing so stores a traceback object directly on the
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Event object, which may cause reference cycles. See the
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<a class="reference external" href="https://docs.python.org/2/library/sys.html#sys.exc_info" title="(in Python v2.7)"><code class="xref py py-func docutils literal"><span class="pre">sys.exc_info()</span></code></a> documentation.</p>
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</dd></dl>
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<dl class="method">
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<dt id="eventlet.event.Event.wait">
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<code class="descname">wait</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#eventlet.event.Event.wait" title="Permalink to this definition">¶</a></dt>
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<dd><p>Wait until another coroutine calls <a class="reference internal" href="#eventlet.event.Event.send" title="eventlet.event.Event.send"><code class="xref py py-meth docutils literal"><span class="pre">send()</span></code></a>.
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Returns the value the other coroutine passed to
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<a class="reference internal" href="#eventlet.event.Event.send" title="eventlet.event.Event.send"><code class="xref py py-meth docutils literal"><span class="pre">send()</span></code></a>.</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">eventlet</span> <span class="k">import</span> <span class="n">event</span>
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<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">eventlet</span>
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<span class="gp">>>> </span><span class="n">evt</span> <span class="o">=</span> <span class="n">event</span><span class="o">.</span><span class="n">Event</span><span class="p">()</span>
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<span class="gp">>>> </span><span class="k">def</span> <span class="nf">wait_on</span><span class="p">():</span>
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<span class="gp">... </span> <span class="n">retval</span> <span class="o">=</span> <span class="n">evt</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
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<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"waited for </span><span class="si">{0}</span><span class="s2">"</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">retval</span><span class="p">))</span>
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<span class="gp">>>> </span><span class="n">_</span> <span class="o">=</span> <span class="n">eventlet</span><span class="o">.</span><span class="n">spawn</span><span class="p">(</span><span class="n">wait_on</span><span class="p">)</span>
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<span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="s1">'result'</span><span class="p">)</span>
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<span class="gp">>>> </span><span class="n">eventlet</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
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<span class="go">waited for result</span>
|
||
</pre></div>
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</div>
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<p>Returns immediately if the event has already
|
||
occurred.</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">evt</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
|
||
<span class="go">'result'</span>
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||
</pre></div>
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